Positive feedback

"There seems to be a change of about 40 parts per million (ppm) in CO2 levels for every 1 °C change in temperature," says [Peter Cox of the University of Exeter]...Since further global warming is inevitable in the near future, it means we're heading for big natural increases in CO2 on top of human-made emissions.

This extra increase will boost global warming in the coming century to about 50 per cent above mainstream climate projections, says Cox, because they only include the effect of CO2 on temperature, and not temperature's effect on CO2.

"The system turns out to be more sensitive than we thought. If we get 4 °C of warming in the coming century, that by itself will raise CO2 levels by an extra 160 ppm.

-- from Rising temperatures bring their own CO2, Fred Pearce's report on Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change.